ATTACHMENT , FACIES DISTRIBUTION, AND LIFE HISTORY STRATEGIES IN CRINOIDS FROM THE UPPER ORDOVICIAN OF KENTUCKY

Late Ordovician crinoids from the Cincinnati Arch region exhibit distinct modes of attachment that may relate to their facies distributions and life history strategies. Certain forms, such as Anomalocrinus, were permanently attached via solid cemented holdfast disks. These crinoids rarely occur in articulated clusters; conversely the lower ends of the columns were frequently partially buried indicating that these crinoids were sufficiently strongly attached that they remained intact through many disruptive events. Such crinoids were hard substrate specialists, but occurred across a range of water depths. Conversely, Ectenocrinus and Iocrinus, were weakly attached to the substrate and mainly relied upon the weight of their columns for temporary anchorage. Such taxa were probably confined to areas below average storm wave base; however, they evidently were subject to major disruption during large storms, as they frequently occur in large, monospecific clusters, with masses of current aligned columns in mudstones. In shallower facies, the distinctive calyx plates and columnals of glyptocrinid camerates form a major constituent of skeletal sediments. Recently discovered beds (Lexington Limestone, Strodes Creek Member; Georgetown, KY) feature masses of non-aligned, articulated crinoids in cross-bedded skeletal grainstone (calcarenite). Stacked obrution deposits occur in multiple foreset beds. Excellent preservation indicates that the crinoids were preserved near their living site, a paradox, as glyptocrinids, lacking a cemented or radicular holdfast, seemingly could not have survived on shifting skeletal sand shoals. One resolution is that even these “high-energy” grainstones were only reworked infrequently. Dense patches of crinoids may have survived in microtopographic lows, e.g., troughs of skeletal sand dunes or channels. The glyptocrinids attached to thickets of bryozoans and each other by the slender tapered distal end of the column, or simply used the recumbent portions of the column itself as an anchor. Despite suffering frequent mass mortalities, loosely attached Ordovician crinoids, presumably survived because of rapid proliferation as r-selected generalists. Disruptive events were sufficiently rare that they did not provide a significant selective factor for firm attachments.